Nucleating agent comprising a silicon dioxide core and injection molding method of polyethylene terephthalate

ABSTRACT

The invention relates to a nucleating agent for polyethylene terephthalate and injection molding method thereof. The nucleating agent comprises a SiO 2  core grafted with bis(2-hydroxyethyl)terephthalate, which accelerates the crystallization of polyethylene terephthalate for injection molding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polyethylene terephthalate (PET), andin particular relates to a nucleating agent and an injection moldingmethod for PET.

2. Description of the Related Art

Contemporary material science has many development branch includingmetals, alloys, ceramics, and polymers. Polymers are most complexthereof because many properties such as viscosity, crystallinity, andmelting point are influenced by types of monomers, degree ofpolymerization, arrangements of the repeating unit, and crosslinkingbetween the polymers. These properties determine the applications andtypes of molding methods of the polymers. Conventional methods ofregulating these properties include altering the monomer to initiatorratio, polymerizing temperature and pressure, and adding nucleatingagents to accelerate crystallization of the polymers.

Polyethylene terephthalate (PET) is inexpensive, however; it cannot beused in injection molding because of its slow crystallization rate athigh temperatures. On the other hand, PET shaped articles formed at lowtemperature (60 to 70° C.) are susceptible to degradation. In U.S. Pat.No. 4,272,475, Chi discloses a method for controlling the cylindertemperature, the nozzle temperature, the injection temperature, and theshot time for PET injection articles, but the molecular weight of thearticles is too low to be used in engineering-grade plastics. Thedescribed problems result from low crystallization rates ofhigh-molecular-weight PET articles at high temperatures. As high moldingtemperature is required for PET melt to crystallize, an oil bath systemis necessary for PET molding. Compared with the water bath system usedfor polybutylene terephthalate (PBT), the oil bath system for PET ismore costly and dangerous.

For increasing the crystallization rate of PET, U.S. Pat. No. 4,451,606discloses an organic nucleating agent, which is a copolymer ofpoly(alkylene terephthalate) and a sulfonate salt of an aromaticdicarboxylic acid. This organic nucleating agent can be miscible withPET and cause uniform nucleation. The organic nucleation agent, however,may lose its function at high temperature (about 280-300° C.) due todisproportion or degradation.

To avoid the problems of the organic nucleating agents, inorganicnucleating agents to improve PET crystallization rate have been adopted.As disclosed in U.S. Pat. No. 5,886,088, suitable inorganic nucleatingagents include talc, mica, wollastonite, clay, kaolin, diatomaceousearth, bentonite, montmorillonite, hydrotalcite, calcium carbonate,titanium oxide, potassium titanate, asbestos, or barium oxide. Theinorganic nucleating agents are less costly, chemically stable,applicable at high temperatures, and of small size to minimize grainradius. The inorganic nucleating agents may improve dimension stabilityand mechanical strength, but they tend to aggregate at high temperaturesdue to their large surface area and fewer coordination sites. Thedescribed aggregation results in non-uniform nucleation of polymer,thereby deteriorating the properties of PET.

BRIEF SUMMARY OF THE INVENTION

A detailed description is given in the following embodiments withreference to the accompanying drawings.

The invention provides an inorganic/organic oligomer reactive nucleatingagent which can be dispersed uniformly in PET by in-situ polymerizationwithout additional fabrication process. The inorganic/organic oligomersreactive nucleating agents overcomes the aggregation of the inorganicnucleating agent, thereby improving crystallization rate and reducinginjection-molding cycle time. With the nucleating agent of theinvention, PET can substitute the expensive polybutylene terephthalate(PBT) to serve as an injection grade engineering plastic material.

The invention provides a nucleating agent of polyethylene terephthalate,comprising a silicon oxide core, and a plurality of bis(2-hydroxyethyl)terephthalate oligomers grafted on the surface of the silicon oxidecore, and the nucleating agent having the formula:

wherein n is an integer larger than 1.

The invention further provides an injection molding method forpolyethylene terephthalate, comprising subjecting terephthalic acid,silicon oxide nanoparticles, and ethylene glycol to esterification toform a bis(2-hydroxyethyl)terephthalate monomer and the describednucleating agent; polymerizing the bis(2-hydroxyethyl)terephthalatemonomer to form the polyethylene terephthalate, wherein the polyethyleneterephthalate and the bis(2-hydroxyethyl)terephthalate oligomers proceedan in-situ polymerization; blending additives with the polyethyleneterephthalate by an extruder and processing the blend into pellets; andfinally, melting the pellets and injecting the melt into a mold, andcooling the mold to form an article.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

The invention provides an inorganic/organic oligomer reactive nucleatingagent. A nanometer scaled SiO₂ (15-25 nm) core is modified by aplurality of bis(2-hydroxyethyl) terephthalate (BHET) oligomers to formthe nucleating agent, having the formula as below, with a diameter ofabout 55-65 nm.

wherein n is an integer larger than 1, preferred from 2-15.

A plurality of BHET oligomers grafted on the SiO₂ surface may increasethe steric effect and the dispersion degree of the nucleating agent.Because the BHET oligomers are original monomers of PET, no additionalprocedure is required. The obstacles from an organic nucleating agent,such as disproportion or degradation at high temperature, are overcomeby the inorganic/organic oligomer reactive nucleating agent.

The invention further provides an injection molding method ofpolyethylene terephthalate. First, terephthalic acid, silicon oxidenanoparticles, and ethylene glycol are subjected to esterification toform a bis(2-hydroxyethyl)terephthalate monomer and a describednucleating agent. The bis(2-hydroxyethyl)terephthalate monomer ispolymerized to form the polyethylene terephthalate, wherein thepolyethylene terephthalate and the bis(2-hydroxyethyl)terephthalateoligomers are polymerized in-situ. Additives are blended with thepolyethylene terephthalate by an extruder, and then the blend isprocessed into pellets. Finally, the pellets are melted, the melt isinjected into a mold, and the mold is cooled to form an article. Generaladditives in this field include chain extenders, solid-statingaccelerators, impact modifiers, nucleation promoters, anti-hydrolysisadditives, reinforcements, flame retardants, or nucleating agents.Because the nucleating agent is grafted to the molecular chain at thebeginning, the additives of the invention do not require nucleatingagents. Amount of the additives can be changed as necessary. The mold iscooled by a water bath system instead of the conventional oil bathsystem, thereby reducing cost and factory accidents. The lower coolingtemperature means shorter cooling time and shorter injection moldingcycle time. The esterification is performed at a temperature of about150 to 200° C., and the polymerization of polyethylene terephthalate isperformed at a temperature of about 250 to 300° C. The described articlehas a molecular weight of about 15000 to 20000, preferably about 18000.The article has a cold-crystallization peak temperature above 200° C.,and semi-crystallizes in less than 1 minute at a temperature below 190°C. During polymerization of the bis(2-hydroxyethyl)terephthalate monomerprocess, the BHET oligomers of the nucleating agent and the PET arepolymerized in-situ, such that the nanometer scaled SiO₂ is uniformlydispersed into the PET. The BHET oligomers grow longer making the BHETchain hard to break at high temperatures, and the larger steric effectof the longer oligomers may improve the degree of dispersion.

Alternatively, the PET raw material is formed first, theinorganic/organic oligomer reactive nucleating agent and the additivesis blended with the PET raw material by an extruder, and the blend isprocessed into pellets. After the described process, the pellets aremelted, and the melt is injected into a mold.

Conventionally, the PET is melted at 280-300° C., and the nucleatingagent is then added for forming pellets by mechanical force. This kindof PET tends to thermally degrade or degrade due to thermal-oxidation athigh temperature processes. On the other hand, the inorganic nucleatingagents are susceptible to aggregate. The invention provides a nucleatingagent which in-situ polymerizes with the monomers, thereby preventingthe obstacles from the conventional nucleating agents and procedures.

Compared to the expansive PBT, the less costly PET with added nucleatingagent of the invention has similar crystallization properties. Themodified PET of the invention can be applied in engineering-gradeplastic, such as electronic components or motor vehicle parts. The SiO₂of the nucleating agent enhances wear-resistance and mechanicalproperties of the PET, such that the PET can substitute conventionalnylon fiber, such as fishing lines, fishing nets, tires, drapes, orother nylon production.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1

Ethylene glycol (63 g, 1 mol), SiO₂ (1.2 g, 19.9 mol), and p-phthalicacid (166 g, 1 mol) were charged in a reactor under N₂ at a backpressure of 3 kg/cm², and then heated to 180-200° C. for distilledwater. The described esterification was completed in about 2.5 hours. Anucleating agent of a diameter of about 59.8 nm was formed afterpolymerization at 280° C. and back pressure less than 60 mmHg. Thermalanalysis shows that the nucleating agent cannot be decomposed over 300°C., which means the nucleating agent has great thermal stability. Thedescribed reaction formula is:

Example 2

BHET oligomers(50 g, 0.2 mol) and SiO₂ (5 g, 0.08 mol) were charged in areaction apparatus, and heated about 1.5 hours (the heating time dependson the polymerization degree of the BHET oligomers). A nucleating agentsimilar to Example 1 was formed by polymerization at 280° C. and backpressure less than 60 mmHg. The described reaction formula is:

Example 3

Ethylene glycol (63 g, 1 mol), various amounts of SiO₂, and p-phthalicacid (166 g, 1 mol) were charged together for polymerization to form themodified PET. Properties of the PET, the PBT, and the modified PET areshown in Table 1.

TABLE 1 PET/ PET/ PET/ PET/ nucle- nucleating nucleating nucleatingating PBT PET agent agent agent agent SiO₂ (wt %) 0 0 0.25 0.5 1 3 Tg (°C.) 45 76.9 74.4 76.5 77.7 76.1 Tm (° C.) 220 249.2 250.1 254.3 253.1259.3 Tcc (° C.) 174.8 167.3 182.5 205 182.0 180.6 crystallinity 8.5 1.62.3 8.5 4.3 5.9 (%)

Accordingly, the modified PET of the invention is similar to the PBT.All modified PET properties are better than PET without nucleatingagent, such as glass transition temperature (Tg), melting point (Tm),cold-crystallization temperatures (Tcc), and crystallity. Compared tothe PET without nucleating agent, the PET added with 0.5 wt % ofnucleating agent reduces the semi-crystallization time from 2.55 minutesto 0.87 minutes. The shorter semi-crystallization time makes modifiedPET similar to PBT, but the raw material cost of the PET is much lessthan PBT.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A nucleating agent of polyethylene terephthalate, comprising: asilicon oxide core, and a plurality of bis(2-hydroxyethyl) terephthalateoligomers grafted on the surface of the silicon oxide core, and thenucleating agent having the formula:

wherein n is an integer larger than
 1. 2. The nucleating agent asclaimed in claim 1, wherein n is from 2 to
 15. 3. The nucleating agentas claimed in claim 1, wherein the silicon oxide core has a diameterfrom about 15 to 25 nanometers.
 4. The nucleating agent as claimed inclaim 1, wherein the nucleating agent has a diameter from 55 to 65nanometers.